Optical packet switching (OPS) and optical burst switching (OBS) are considered as next-generation transport technologies that will enable more efficient and flexible utilization of optical network capacity by providing sub-wavelength granularity. Optical label switching is a promising packet switching scheme that offers a number of features that are important to future data-intensive networks such as protocol and format independent transparency at the all optical data plane, simplification in the control plane and interoperability with legacy and emerging networking technologies including Generalized Multiprotocol Label Switching (GMPLS). Such a switching scheme was discussed by D. J. Blumental et al in a publication entitled “All-Optical Label Swapping Networks and Technologies”, which appeared in the Journal of Lightwave Technologies, Vol. 18, pp. 2058-2075 in 2000, the entire contents of which are incorporated herein by reference.
One optically labeled packet transmission based on an orthogonal intensity modulation/differential phase shift keying (IM/DPSK) modulation format—in which payloads are intensity modulated while labels are carried by DPSK—has been proposed and demonstrated. (See., e.g., T. Koonen, et al, “Optical Packet Routing In IP-Over-WDM Networks Deploying Two-Level Optical Labeling”, Proc. Eur. Conf. Optical Communications (ECOC 2001), pp. 14-15, 2001; N. Chi, et al, “Dispersion Management for Two-Level Optically Labeled Signals in IP-Over-WDM Networks”, Proc. Eur. Conf. Optical Communications (ECOC 2002), Paper 5.5.1, Copenhagen, Denmark, 2002; T. Koonen et al, “Optical Labeling Of Packets in IP-over-WDM Networks” Proc. Eur. Conf. Optical Communications (ECOC 2002), Paper 5.5.2, Copenhagen, Denmark, 2002; and N. Chi etal, “Transmission and Transparent Wavelength Conversion of an Optical Labeled Signal Using ASK/DPSK Orthogonal Modulation”, IEEE Photon. Tech. Lett., vol 15, pp. 760-762, May 2003).
Unfortunately, a number of significant drawbacks characterize these schemes. For example, since DPSK is carrying low-speed label information, a delay interferometer—which is needed in DPSK demodulation—produces a long delay difference between the two arms of the delay interferometer thereby resulting in significant temperature and polarization sensitivities. In addition, any change in the data rate of the label requires a change to the delay of the interferometer (to obtain a matched delay).
A further characteristic of such prior art schemes is that the removal and insertion of a label is not easily performed since the label is phase modulated. More specifically, to remove a label, cross gain modulation is used which changes the signal wavelength and therefore is undesirable in situations where signal wavelength needs to be maintained. To add a label, expensive phase sensitive modulation is required. Finally, low speed DPSK requires a high-coherence (narrow linewidth) source (CW) laser and is less tolerant to nonlinear effects such as cross-phase-modulation (XPM).
Other schemes utilizing frequency shift keying (IM/FSK) (See, e.g., G. K. Chang et. al., “Multirate Payload Switching Using a Swappable Optical Carrier Suppressed Label In a Packet-Switched DWDM Optical Network”, J. Lightwave Technol., vol 23, pp. 196-202, January 2005), subcarrier modulation (IM/SCM) (See, e.g., M. Hickey et. al. “Combined Frequency and Amplitude Modulation for the STARNET WDM Computer Communication Network”, IEEE Photon. Technol. Lett., Vol. 6, pp. 1473-1475, December 1994), or polarization shift keying (PoISK/IM) (See, e.g., C. W. Chow, et. al., “Optical Packet Labeling Based on Simultaneous Polarization Shift Keying and Amplitude Shift Keying”, Optics Letters, Vol. 29, pp. 1861-1963, August 2004) have also been demonstrated.
More recently, it has been shown that using DPSK/IM for payload/label modulation and using a balanced receiver for DPSK detection provides superior receiver sensitivity of around −36 dBm for both the label and payload at a bit error rate (BER) of 10−9. (See, e.g., X. Liu, et. al., Optical-Label Switching Based on DPSK/ASK Modulation Format With Balanced Detection For DPSK Payload”, Proc. Eur. Conf. Optical Communications (ECOC 2003), Paper Tu4.4.3, Rimini, Italy, 2003; and X. Liu, et. al., “Transmission of an ASK-Labeled RZ-DPSK Signal and Label Erasure Using a Saturated SOA”, IEEE Photon. Technol. Lett., Vol 16, pp. 1594-1596, June 2004).
In a majority of these optical label encoding schemes, two optical modulators are required, one for encoding the payload and the other for the label. Consequently, such schemes are not particularly cost-effective and not easy to work with since the polarization state of the signal needs to be constantly maintained and its alignment when entering the two optical modulators is critical.